1. Field of the Invention
The present invention relates generally to a method of manufacturing a glass product and is directed more particularly to a method of manufacturing an asymmetrical glass panel used in, for example, a flat type cathode ray tube.
2. Description of the Prior Art
At present, there has been proposed such a flat type cathode ray tube in which as shown in FIGS. 1 and 2 an electron gun is located to extend in the horizontal or vertical direction along the surface direction of the phosphor screen so as to make the cathode ray tube flat.
In FIG. 1 which is a plan view of the flat type cathode ray tube and in FIGS. 2A to 2D which are each a cross-sectional view taken along the line II--II in FIG. 1 and show different cathode ray tubes, reference numeral 1 generally designates the flat type cathode ray tube and 2 its panel portion.
FIG. 2A shows such a case where the panel portion 2 is divided into two panel members 2a, 2a which are substantially the same in configuration with respect to the vertical plane passing through a line VL, while FIG. 2B is the case that the panel portion 2 is divided into two panel members 2a, 2b which are different with each other in configuration with respect to the vertical plane including the line VL.
As shown in FIG. 3A, one of the divided panel members i.e. panel member 2a has a U-shaped cross-section, its panel surface is curved in one direction, it is symmetrical with respect to the vertical plane passing through a line a--a (which corresponds to the line II--II in FIG. 1) but it is asymmetrical with respect to the vertical plane passing through a line b--b perpendicular to the line a--a. In case of FIG. 2B, although the panel member 2a is approximately same as that 2a of FIG. 2A, as shown in FIG. 3B, the other panel member 2b has a U-shaped cross-section and its panel surface somewhat flat or approximately straight line shaped, it is symmetrical with respect to the vertical plane including the line a--a but asymmetrical with respect to the vertical plane including the line b--b. FIG. 2C shows a case where the panel portion 2 is divided into a panel member 2a' which is substantially the same as that 2a shown in FIG. 2A in shape but somewhat deeper than the latter in inner height (refer to FIG. 3C) and into a plate-shaped panel member 2 b' (refer to FIG. 3D). FIG. 2D shows a case where the panel portion 2 is not divided and is of a unitary shape as shown in FIG. 4.
In FIGS. 1 and 2, reference numeral 3 denotes a phosphor screen, 4 a funnel portion, 5 an electron gun, 6 welding portions along which the divided panel members 2a, 2b and 2a', 2b' or panel portion 2 and the funnel portion 4 are respectively made together by crystallizing a vitreous solder glass and 7 which is an anode button through which a high voltage is applied to the cathode ray tube.
As the flat type cathode ray tube, it is an important matter to make its panel portion 2 flat. Although the panel portion 2 of unitary or integral structure can achieve the purpose to make the panel portion 2 flat, it is rather difficult to manufacture the panel portion 2 of integral structure. Firstly, according to the method of molding the panel portion 2 of the integral structure by using the metal press, since the bottom mold thereof is deep, if the temperature of the glass gob is selected higher than that of a glass gob used to make ordinary molded products, the raw glass material can not reach the top end of the bottom mold and the product with a thin thickness is difficult to be made. The fact that the increase in temperature of the glass gob i.e. the increase in the metal mold easily causes the surface of the metal mold to be chapped (including oxidization), so that the inner and outer surfaces of the panel portion thus made are roughened. It is rather easy to grind or polish the outer surface of the panel portion but it is difficult to grind or polish the inner surface of the panel portion and hence it requires much work. Accordingly, in case of the panel of a cathode ray tube of very flat type, since the roughess on the inner surface thereof influences or affects on the permeability of light, such a panel, namely panel of integral type and deep structure is not suitable for massproduction.
In case of the glass panel portions 2a, 2b and 2a' shown in FIGS. 3A, 3B and 3C, they are all asymmetrical with respect to the vertical plane including the line b--b. Thus, when the glass panels with such configuration are made directly by the press mold, there may occur such a problem that after the press mold of glass by a metal mold, owing to the anisotropy in the thermal distortion of the product upon cooling, the dimensional error is large, scattering appears in its quality, hence its productivity is poor and its manufacturing cost is high.
In general, it is said that a body with a dish-shape and symmetrical with respect to two planes perpendicular to each other, a body with a shape symmetrical with respect to an axis or a point, or body with a shape similar to the foregoing is easily made and the productivity thereof is good.